In normal subjects during exercise, there is vasoconstriction of non-exercise resistance vessels and an increase in blood pressure. We have investigated patients with vasovagal syncope with structurally normal hearts and patients with hypertrophic cardiomyopathy compared with normal controls and found a failure of vasoconstriction in both patients with vasovagal syncope and hypertrophic cardiomyopathy compared with normals and this has been associated with exercise hypotension. An association between exercise hypotension and sudden death has previously been reported in hypertrophic cardiomyopathy. We speculate that in patients with vasovagal syncope and structurally and electrically normal hearts, exercise hypotension is well tolerated, but in patients with structurally abnormal hearts, exercise hypotension can have catastrophic consequences.